The present invention relates generally to a wireless communications system including an airborne repeater, and particularly to a smart antenna for use in such a system that is capable of beam steering and shaping and that compensates for motion of an airplane.
The increasing need for communications networks and capabilities in outlying and geographically diverse locations has created greater demand for cellular systems. Many new carriers providing the infrastructure for such systems have focused their resources on building as many terrestrial cell stations as possible to expand their respective areas of coverage and consequently generate more revenue.
However, the buildout rate for the terrestrial cell stations is typically slow and expensive, especially in mountainous or otherwise difficult to access areas. In addition, in some of these areas, a carrier""s return on investment may not provide the incentive necessary for the carrier to build the necessary cell stations, thereby leaving these areas with either limited or no cellular service at all. Further, many areas having a sufficient number of cellular communications base transceiving stations to handle calls during both off-peak and peak times cannot adequately handle large volumes of calls during sporting events or other short-term special events that temporarily attract large crowds.
In response to the above, airborne cellular systems have been proposed in which a cellular repeater mounted in an airplane, flying a predetermined flight pattern over a geographic area, links calls from cellular phones within the covered geographic area to a terrestrial base station. Because the airplane is capable of traversing geographic limitations and takes the place of the cell stations, such a system overcomes the above-mentioned limitations of conventional terrestrial cellular systems.
Despite its many advantages, an airborne cellular system presents design and implementation problems not present in the design and implementation of conventional terrestrial cellular systems. For example, as the airplane circles in its flight pattern, communications beams radiated from the airplane antenna move relative to the ground, thereby causing the system to perform call handoffs as beams rotate into and out of predetermined system areas of coverage. In addition, cellular systems adjacent to the airborne system present potential beam interference issues. Large call loads in certain areas and small call loads in other areas also tend to require an airborne system to provide more power, and consume more radio spectrum, than would be necessary if the call loads in each area were balanced. In addition, multipath Doppler and delay spread within an airborne system depends on the underlying terrain characteristics and the speed of the aircraft and are more pronounced than in a conventional terrestrial cellular system, which may reduce the performance of existing user handsets. Also, variations in airplane pitch, roll and yaw can move communications beams off-target and result in interference with other cellular systems and therefore in system non-compliance with FCC regulations. Further, nonuniform subscriber density results in less efficient use of spectrum because the spectral capacity of each beam must be sized for the maximum density region. Clearly a need exists for solution to the foregoing problems.